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Simple robots that toddle along like an old-fashioned child's toy offer a more realistic and efficient model of human walking than more sophisticated models, researchers say.

They hope their back-to-basics approach can be used not only to design more efficient robots, but also prosthetics for injured patients and amputees, and to understand better how people walk.

The idea is based on 'passive-dynamic walkers', devices that can walk down a slight slope using only gravity and carefully balanced, pendulum-like legs.

These unpowered walkers can produce a surprisingly human-like gait, three separate teams of researchers report in today's issue of the journal Science.

Adding a tiny bit of power, as much as is used by a small fluorescent light bulb, allows an element of control for the walker to make more than a few steps and adjust to differing terrain, including level ground.

"We can let the mechanics take care of a lot of the motion as opposed to motors," says Professor Andy Ruina of Cornell University in New York, who helped to design one of the robot walkers.

This approach gives a more realistic gait than fully powered robots, and is much more energy-efficient, which will save batteries in the long run.

The concept is simple, says Ruina. The legs act like sticks attached to hinges, and swing back and forth with a pendulum motion. The concept has been used for more than 100 years to design toys that 'walk' down a slope with no propulsion.

But mainstream robots have every movement carefully controlled and powered.

Steven Collins of Cornell, who is now at the University of Michigan, adds a tiny bit of motorised propulsion at the robot's ankles.

"At each step it pushes off with its back foot," Collins says. "This is similar to how we think people walk."

Adding a hinged knee allows the robot to clear obstacles or step uphill.

Dutch researcher Dr Martijn Wisse of the Delft University of Technology adds pneumatic powering to simulate muscles in the legs of his robot, which uses very similar principles.